The invention relates to a blowing device of sheet deliverers of sheet processing machines, particularly printing machines. In sheet processing machines, for example in printing machines, the sheet deliverer will remove the sheets after printing by means of gripper carriages, transport these over a stack where they are released and placed upon the stack. Forming of the stack is aided by pneumatic systems, used preferably with high-speed machines and large sheet sizes.
By the patent application designated DD-WP B 65 h /208,953, a pneumatic device to aid in stack forming has become known. The device consists of longitudinal blow tubes above the stack, by which the sheet to be stacked is partially impinged from above by blown air, this in order to expel the air cushion between the upper edge of the stack and the sheet. Suction elements are, furthermore, provided at the lateral zones of the rear edge of the stack, serving to evacuate the air between the stack and the sheet. The suction elements serve for the rapid moving of the rear edge of the stacking sheet out of the sheet path, since the former can be influenced only to an insufficient degree by the air blown at the top. This device has the disadvantage that the relative motion between the stacking sheet and the transported sheet above it will cause a reduction of air pressure between these sheets relative to the prevailing air pressure below the stacking sheet, thus causing a reduction in the fall velocity of the stacked sheet, this pressure differential being difficult to remove.
In consequence of the pressure differential still remaining, the end of the stacking sheet may even be pulled towards the transported sheet and be dragged along by the latter so that no exact stack can be formed.
Furthermore, the suction chests will evacuate air from a space which is not clearly defined, since air from the free space may flow in at the particularly endangered sheet corners, thus reducing the efficacy of the device. The further disadvantage of this device is that it can be produced and operated only at relatively high expenditure.